Restore asserts and error handling

A couple of them got improved out of existence.

src/statesync/user.cc

@@ -95,6 +95,7 @@ string UserStream::diff_from( const UserStream &existing ) const
       }
       break;
     default:
+      assert( !"unexpected event type" );
       break;
     }
 
@@ -130,6 +131,7 @@ const Parser::Action &UserStream::get_action( unsigned int i ) const
   case ResizeType:
     return actions[ i ].resize;
   default:
+    assert( !"unexpected action type" );
     static const Parser::Ignore nothing = Parser::Ignore();
     return nothing;
   }

src/statesync/user.h

@@ -85,7 +85,7 @@ namespace Network {
     /* interface for Network::Transport */
     void subtract( const UserStream *prefix );
     string diff_from( const UserStream &existing ) const;
-    string init_diff( void ) const { return string(); };
+    string init_diff( void ) const { return diff_from( UserStream() ); };
     void apply_string( const string &diff );
     bool operator==( const UserStream &x ) const { return actions == x.actions; }